Proton impact ionization and a fast calculation method

Abstract

We present a coupled Monte Carlo and multistream model simulating primary ionization and secondary electron ionization, respectively, from energetic proton precipitation in the Earth's upper atmosphere. Good agreement is obtained with previous model results. It is found that while secondary electrons make a negligible contribution to ionization from low‐energy (≤10 keV) auroral proton precipitation, their importance increases with increasing incident proton energy, confirming earlier findings. It becomes significant or even comparable to primary ionization from protons and generated hydrogen atoms in charge‐changing collisions. Our calculations of the mean energy loss per ion pair production show a nearly monotonic increase with incident proton energy, ranging from about 22 eV to 33 eV when incident energy increases from 100 eV to 1 MeV. To facilitate a fast calculation in large‐scale computations, we develop a parameterization for total (primary plus secondary) ionization from monoenergetic proton precipitation. This is obtained by fitting to a large set of numerical results from the coupled model. The quick method applies to a wide energy range of 100 eV to 1 MeV for incident monoenergetic protons, and its validity has been extensively tested under a variety of background atmospheric conditions. Our new parameterization can be used to rapidly calculate the ionization altitude profile from precipitating protons with any spectral distributions without any significant compromise in accuracy. By considering branching ratios of ionized atmospheric species, the fast calculation method is thus useful for self‐consistently including proton impact effects in large community models.